Two-dimensional crystals of colloids are widely used as templates for the structuring of surfaces. Among others, they may serve as lithographic masks for etching or sputtering processes, as patterned arrays of relief structures to cast elastomeric stamps for use in soft lithographic techniques, as photonic crystals and microlenses.
Especially lithography benefits from the use of colloidal crystals as their production is usually simple, cost effective, allows high throughput and supersedes the use of complex equipment, thus having great advantages over conventional lithographic approaches.
A vast of techniques has been developed to produce 2D colloidal crystals. However, only a relatively small number of techniques is focused on the fabrication of non-close packed colloidal crystals (Jiang, P., et al., Appl. Phys. Lett. 89, 011908-3 (2006); Zhang, G., et al., Chemistry of Materials 17, 5268-5274 (2005); Li, X., et al., Langmuir 26, 2930-2936 (2010)). The developed techniques provide only low throughput, apply only to small scale surfaces or involve at least a post-treatment of the colloidal crystal such as reactive ion etching. Consequently, these techniques of the art undermine to some extend the aforementioned advantages of colloidal crystals.
It is principally known in the prior art that self-assembling poly-N-isopropylamide (polyNIPAM) microspheres can be used to produce two-dimensional colloidal arrays, with the superlattice structure of these colloids being controllable by designing the particle structure (Tsuji and Kawaguchi, Langmuir 2005, 21, 2434-2437). Tsuji and Kawaguchi prepared the colloidal arrays by dropping an aqueous PNIPAM microgel dispersion onto various substrates and air-drying. Experiments of the present inventors revealed that these conditions resulted in an uneven distribution of the deposited microgel particles and both the near-range and the long-range order of the colloidal arrays obtained by this method was not satisfying. The uneven distribution of the deposited microgel particles is based on the increasing concentration of particles in the dispersion upon drying. Therefore the two-dimensional array shows a lot of defects.
WO 2010/099805 A1 discloses a method for preparing highly ordered arrays of nanoholes in metallic films wherein an ordered array of polyNIPAM microspheres is deposited on a substrate surface and subsequently used as a mask for the nanohole array. Said arrays of polyNIPAM microspheres show a considerably higher degree of order as compared to arrays known from previous prior art, however, it is still not possible to obtain 2D crystal arrays with an exceptional high long range order, including monocrystalline domains in the range of square millimeters, by this method.
Thus, an object of the present invention is to provide improved methods for producing highly ordered arrays of 2 D crystals on a substrate which are fast, cost-efficient and simple to perform without the need of expensive equipment, for example in any standard chemical laboratory. A further object is to provide large and highly ordered arrays of 2 D crystals on a substrate, with the size and lattice constant of the domains being easily adjustable over a broad range.
Said objects are achieved according to the present invention by providing a novel method for producing highly ordered arrays of 2D crystals on a substrate according to claim 1 and by providing the highly ordered array of 2D crystals according to claim 13. Further aspects and preferred embodiments of the invention are the subject of additional claims.